1. Field of the Invention
The present invention relates to a plane carbon commutator used as a commutator for a motor of a fuel pump and the like, and to a producing method of the plane carbon commutator. More particularly, the present invention relates to a plane carbon commutator fore reliably connect segment and carbon in the commutator, and to a producing method of the plane carbon commutator.
2. Description of the Related Art
A plane carbon commutator comprises a metal segment attached to an end face of a commutator body made of mold resin, and carbon attached to the segment. As a producing method of this kind of plane carbon commutator, there are the following methods (A) to (D) for example.
(A) When carbon is formed, a base metal which is a segment is inserted into the carbon and the base metal and the carbon are integrally formed and burnt and then, the base metal which was integrally molded with the carbon is integrally molded with the mold resin, thereby forming an insulator portion (see Japanese Patent Application Laid-open No. H7-264812 for example).
(B) An insulator and a metal base are previously integrally formed by integrally molding or the like and then, carbon is attached to a face of the metal base by soldering or conductive adhesive (see Japanese Patent Application Laid-open No. H5-502974 for example).
(C) An insulator and a metal base are previously integrally formed by integrally molding or the like and then, carbon is formed on a face of the metal base and burnt (see Japanese Utility Model Publication No. H7-42223 for example).
(D) An insulator and a metal base are previously prepared separately, and when carbon is formed, the metal base and the insulator are integrally molded with the carbon (see Japanese Patent Application Laid-open No. H6-178503 for example).
In the method (A), a temperature for burning the carbon is high as higher as about 600xc2x0 C. or higher. Therefore, the integrally formed metal base is softened, and the product has a problem in terms of precision and strength. Thereupon, the carbon can be burnt at a low temperature about 200xc2x0 C., but in such a case, the quality of material of the carbon itself becomes special, and there is a problem in various characteristics such as hardness, electrical resistance and gasoline resistance.
In the method (B), the carbon can be previously burnt singly, and there is no problem in the quality of material of the carbon itself. However, if the carbon is soldered to the face of the metal base, there is an adverse possibility that the solder is loosened by a high temperature of wire at the time of assembling of a motor.
If the metal base and the carbon are adhered by the conductive adhesive, this structure requires an adhesive having both conductive property and gasoline resistance, which is expensive. Further, even if the adhesive has the conductive property, electrical resistance thereof is greater as compared with the carbon and the metal base, and there is a problem that this portion is prone to generate heat and a material thereof is prone to be changed when a motor is driven.
In the method (C), the insulator made of resin is carbonized by a high temperature when the carbon is burnt on the face of the metal base. Therefore, the carbon must be burnt at a low temperature, and there is a problem in the quality of material of the carbon.
The method (D) has the same problem as that of the method (C).
Thereupon, in order to solve the above-described conventional problems, the present assignee filed Japanese Patent Application No. H9-51991 (Japanese Patent Application Laid-open No. 10-4653, which will be referred to as xe2x80x9cprior examplexe2x80x9d hereinafter). In the prior example, a segment is formed with an engaging hole, and an engaging projection provided with carbon that was previously burnt at a high temperature is engaged into the engaging hole and integrally formed as one piece. Therefore, the segment and the carbon can be integrally formed with out using solder or adhesive, and the initial object could be achieved.
In the prior example, when the engaging projection formed on the carbon is engage with the engaging hole formed on the segment and attached and integrally formed, shrinkage fit and press-fit are carried out, and after the engaging projection is engaged with and inserted into the engaging hole, burring or the like is carried out. The engaging projection is fastened by a projection formed by the burring.
Therefore, when the engaging projection of the carbon is inserted into the engaging hold of the segment and both of them are integrally formed, there is a problem that the process for integrally forming them is troublesome.
It is an object of the present invention to solve the problems described above.
According to a first aspect of the present invention, there is provided a plane carbon commutator comprising a plurality of metal segments fixed to a commutator body made of resin, engaging projections provided on a carbon which was previously burnt at high temperature, said engaging projections being engaged with engaging holes provided in said segments and integrally formed as one unit, wherein tip ends of cut-rising pieces functioning to allow insertion of said engaging projections into said engaging holes but prevent said engaging projections from being pulled out from said engaging holes, and said cut-rising pieces are brought into contact under pressure from peripheral faces of said engaging projections.
According to a third aspect of the present invention, in the plane carbon commutator of the first or the second aspect, conductive paste is interposed between said segments and said carbon.
According to a fourth aspect of the present invention, there is provided a producing method of a plane carbon commutator comprising a plurality of metal segments fixed to a commutator body made of resin, and carbon, said segments and said carbon are integrally fixed to each other, wherein said method comprises the steps of: (a) forming peripheral faces of said engaging projections into coarse faces when engaging projections formed on said carbon are inserted into engaging holes formed in a metal base which will become said segments in order to integrally form said carbon which was previously burnt at a high temperature and said metal base; (b) integrally forming said metal base and said carbon and then, coating the entire exposed face of said carbon with mold resin when said engaging projections formed on said carbon are inserted into said engaging holes formed in said metal base; (c) cutting said metal base into each segment and at the same time, cutting said carbon; and (d) removing said mold resin from a contact face between said carbon and a brush.